linux-kernel - Re: [PATCH v2 2/2] x86: add prefetching to do

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Message-ID: <20131108200803.GD16052@hmsreliant.think-freely.org>
Date:	Fri, 8 Nov 2013 15:08:03 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Joe Perches <joe@...ches.com>
Cc:	Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org,
	sebastien.dugue@...l.net, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v2 2/2] x86: add prefetching to do_csum

On Fri, Nov 08, 2013 at 11:17:39AM -0800, Joe Perches wrote:
> On Fri, 2013-11-08 at 14:07 -0500, Neil Horman wrote:
> > On Fri, Nov 08, 2013 at 08:51:07AM -0800, Joe Perches wrote:
> > > On Fri, 2013-11-08 at 11:25 -0500, Neil Horman wrote:
> > > > On Wed, Nov 06, 2013 at 12:07:38PM -0800, Joe Perches wrote:
> > > > > On Wed, 2013-11-06 at 15:02 -0500, Neil Horman wrote:
> > > > > > On Wed, Nov 06, 2013 at 09:19:23AM -0800, Joe Perches wrote:
> > > > > []
> > > > > > > __always_inline instead of inline
> > > > > > > static __always_inline void prefetch_lines(const void *addr, size_t len)
> > > > > > > {
> > > > > > > 	const void *end = addr + len;
> > > > > > > ...
> > > > > > > 
> > > > > > > buff doesn't need a void * cast in prefetch_lines
> > > > > > > 
> > > > > > Actually I take back what I said here, we do need the cast, not for a conversion
> > > > > > from unsigned char * to void *, but rather to discard the const qualifier
> > > > > > without making the compiler complain.
> > > > > 
> > > > > Not if the function is changed to const void *
> > > > > and end is also const void * as shown.
> > > > > 
> > > > Addr is incremented in the for loop, so it can't be const.  I could add a loop
> > > > counter variable on the stack, but that doesn't seem like it would help anything
> > > 
> > > Perhaps you meant
> > > 	void * const addr;
> > > but that's not what I wrote.
> > > 
> > No, I meant smoething like:
> > static __always_inline void prefetch_lines(const void * addr, size_t len)
> > {
> > 	const void *tmp = (void *)addr;
> > 	...
> > 	for(;tmp<end; tmp+=cache_line_size())
> > 	...
> > }
> > 
> > > Let me know if this doesn't compile.
> > > It does here...
> > Huh, it does.  But that makes very little sense to me.  by qualifying addr as
> > const, how is the compiler not throwing a warning in the for loop about us
> > incrementing that same variable?
> 
> Because it points to const data but is not const itself.
> 
> void * const foo;	/* value of foo can't change */
> const void *bar;	/* data pointed to by bar can't change */
> const void * const baz; /* Neither baz nor data pointed to by baz can change */
> 
Doh!  Wow, that was just staring me in the face and I missed it :)

Thanks for pointing it out.  I'll make that adjustment
Neil

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